Power control is used on the uplink link in a wireless communication system to control the power of signals received at each base station from the wireless devices. As a wireless device moves within the network, the channel conditions change continuously due to fast and slow fading, shadowing, number of users, external interference, and other factors. Closed loop power control algorithms dynamically control the transmit power of the wireless device on the uplink link. Closed loop power control includes inner loop and outer loop power control mechanism. For inner loop power control, the base station measures the SIR of the received signal, compares the measured SIR to a SIR target, and adjusts the transmit power of the wireless device depending on the comparison. Outer loop power control adjusts the SIR target for the inner loop power control mechanism to maintain desired performance criteria, such as a desired Frame Error Rate (FER).
The inner loop power control mechanism provides improved performance for fast fading channels. Typically, in inner loop power control, the base station can send as many as 1500 up/down power control commands per second to the wireless device. The use of up/down power control commands keeps the received power level constant at the base station. When the received power level at the base station remains stable, the number of re-transmissions by the wireless device due to transmission errors can be maintained, e.g., below a threshold. Power control can also reduce intra-cell interference between uplink transmissions.
One problem with power control is that when fast fading occurs, the power of the wireless device may be increased many decibels (dBs) to compensate the path loss due to fast fading. The increase may be as large as 30 dBs. Because the fast fading loss to different cells has low correlation, a large increase of transmit power by the wireless device to maintain the signal level at the serving base station may result in significant interference with neighboring cells.
Therefore, the conventional method of increasing a wireless device's uplink transmit power to compensate for fast fading leads to strong inter-cell interference. The affected neighboring cells may need to combat the interference with additional resources. Improved methods and apparatus are needed for efficient utilization of resources and improved system capacity.